An ESR study on the heterogeneity of dynamics in a nematic polymer inducedby thermal annealing in the isotropic melt

Heterogeneities induced in a nematic polymethacrylate by thermal annealing in the isotropic phase were investigated by studying the dynamics of the ch olestane spin probe dissolved in the host matrix by electron spin resonance . The molecular site distribution was well described by a two delta -like d istribution function. The temperature dependences of the dynamics of the sl ow and fast components were fully characterized in going from the isotropic state to the glassy state through the nematic phase. It was found that the behaviors of the spinning correlation times with temperature in the isotro pic and nematic regions were well represented by the Vogel-Fulcher law. By comparing the temperature dependence of the probe dynamics with the a relax ation process of the polymer, we estimated the cooperativity degrees in the dynamics of the molecular probe in the different sites. The relative popul ations of the slow and fast sites were determined over the whole temperatur e range investigated. It was shown that such population was sensitive to co nformational change of the polymer backbone.